2. In another work, Pramatarova et al. reported a similar result: "Neuron-specific expression of mutant superoxide dismutase 1 in transgenic mice does not lead to motor impairment" (Pramatarova et al., 2001). The authors concluded that the accumulation of mutant SOD1 in postnatal motor neurons is thus not sufficient and probably also not critical to induce or accelerate motor neuron disease in FALS mice.

3. These studies suggest/propose that mutant SOD1 causes the degeneration of motor neurons by a combination of cell-autonomous and non-cell-autonomous processes, requiring the presence of mutant SOD1 in both neurons and glia, and raising the question of whether mutant SOD1 expression in neurons is sufficient to induce disease.

4. More recently, another group performed a similar experiment: Jaarsma et al. (Jaarsma et al., 2008) have demonstrated that transgenic mice in which mutant "SOD1 was largely restricted to neurons," under the transcriptional control of Thy1.2 promoter, "developed disease but only at an old age." However, "the disease progressed slowly without reaching the same degree of paralysis compared to the classical animal model of ALS in which the same mutant SOD1 gene is ubiquitously expressed."

So, what is the truth? These studies do not indicate a clear answer, but suggest that the situation is more complicated and the toxicity is not just limited to a single cell type, since in ALS patients and ALS animal models, the mutated toxic form of SOD1 is expressed in several tissues and not just limited to motor neurons.

What About Skeletal Muscle?
This issue has been recently investigated by our laboratory, which has demonstrated that muscle-selective expression of SOD1 mutation causes pathological alterations and induces presymptomatic sign of ALS (Dobrowolny et al., 2008). Moreover, other studies support the evidence that skeletal muscle is a primary target of mutant SOD1 toxicity in mice. Wong and Martin have reported that skeletal-muscle-restricted expression of the human mutant SOD1 gene causes motor neuron degeneration in old transgenic mice (Wong and Martin, 2010). Dupuis et al. (Dupuis et al., 2009) have reported that muscle-selective alterations in mitochondrial function might initiate NMJ destruction, which is followed by distal axonopathy, astrocytosis in the spinal cord, and mild motor neuron loss. Moreover, Zhou et al. (Zhou et al., 2010) have reported that alterations in the potential of the mitochondrial inner membrane of fiber segments near NMJs occur in young SOD1G93A mice prior to disease onset. All of the above suggests that skeletal muscle is an important candidate to consider as a primary target of the toxicity that results from mutations in the SOD1 gene.

In the paper by Cleveland and coworkers, the authors reported that sustained mitochondrial biogenesis and muscle function do not extend survival in a mouse model of inherited ALS. The authors concluded: "Our evidence refutes such a conclusion, demonstrating to the contrary that sustained improvement in muscle activity, including a doubling in endurance, increased energy supply from the mitochondria in muscles, and reducing muscle atrophy throughout ALS-like disease does not prevent or delay retraction of the axons from neuromuscular junctions, loss of motor axons, or death of motor neurons."

This is, to me, a bias of the authors.

To my knowledge I do not think that anybody succeeded to cure an ALS mouse model simply by acting on motor neurons, but this does not mean that motor neurons are not primary targets of mutant SOD1 toxicity.

Moreover, several studies demonstrated that the toxicity of mutant SOD1 is associated with severe alterations (at morphological, functional, and molecular levels) of the tissue in which it is expressed, and since the selective expression of mutant SOD1 induces muscle atrophy, muscle mitochondrial dysfunction, reduced muscle strength, muscle damage, and presymptomatic signs of ALS in the spinal cord of MLC/SOD1G93A mice (Dobrowolny et al., 2008), this is sufficient to me to conclude that skeletal muscle is a primary target (not the sole) of SOD1-mediated toxicity.

In addition, the fact that increasing PGC-1α activity in the muscles of SOD1 mutant expressing mice produces significantly increased muscle endurance, reduced atrophy, and improved locomotor activity, even at late stages of disease, suggests that the toxic properties of mutant SOD1 in the muscle can be significantly reduced. Of course, this is not sufficient to extend the survival of the ALS mouse model, because the mutant SOD1 protein is also expressed in the spinal cord of transgenic animals, and this is, in my opinion, sufficient to exert its toxic properties.

Nevertheless, in the paper by Cleveland and coworkers, the authors concluded that “improving muscle activity and reducing atrophy may be effective to improve or preserve daily functioning and quality of life for ALS patients.” This is a good starting point to design more appropriate therapeutic strategies to treat ALS patients.

Putting together all of this information, I think:

1. the improvement in muscle mass and function is not sufficient to cure/attenuate significantly the progression of the disease (unless the muscle produces neurotrophic factors, such as IGF-1, that might activate survival pathways at the level of spinal cord; see Dobrowolny et al., 2005; Kaspar et al., 2003);

3. ALS is a multisystemic disease which involves different cell types and tissues, and, in my opinion, we can really attenuate the progression of the disease if we can start to consider using a combinatorial therapeutic approach, acting on motor neurons, glia, and muscle.

I think that these results clearly demonstrate that there is a dissociation between the health of muscle fibers and the denervation process. The former can be prevented by activating mitochondrial biogenesis, but the latter cannot. The conclusion is that enhancing mitochondrial biogenesis exclusively in muscle does not help the motor neurons to maintain their contact with the muscle fibers. There is no benefit on survival, but there is likely an improvement in the quality of life (protection of muscle strength).

There are some intriguing questions that are not discussed in the manuscript.

1. Is SOD1 in muscle the cause of muscle atrophy, or is that due to loss of innervation? The results appear to suggest that the former is the case, and that mitochondrial biogenesis in muscle can prevent it. However, it is unclear how this relates to the previous study in which the authors knocked down SOD1 in muscle only.

2. If muscle mass is preserved, what is the nature of the weight loss that is used as a marker of disease onset in this study. Has lipid metabolism changed, and have the mice lost body fat? Could something else be going on?

3. The paper does not explore the effects of PGC-1α expression in the CNS. Would that be protective? If a pharmacological approach to enhance mitochondrial biogenesis was to be attempted in patients, it would likely have systemic effects, not limited to muscle.

The recent article by Da Cruz et al. is very exciting in that it supports the concept that skeletal muscle is a primary site of toxicity of ALS-linked mutant SOD1 (Dobrowolny et al., 2008; Wong and Martin, 2010). Moreover, the paper by Da Cruz et al. concludes, as did earlier papers (Dobrowolny et al., 2008; Wong and Martin, 2010), that skeletal muscle could be a tissue target for disease-modifying or palliative therapy in human ALS. Thus, this more recent work vindicates these earlier experiments. The enforcement of PGC-1α, a key regulator of mitochondrial biogenesis and function, in skeletal muscle, however, did not extend survival of the mutant SOD1 mice. This is encouraging news, and not contrary to the skeletal muscle hypothesis for ALS mechanisms, because it suggests that the mechanisms of disease in mutant SOD1 in skeletal muscle are not likely to be driven by mitochondrial pathobiology. The paper by Da Cruz et al. fails to rule out many other possible mechanisms of disease in skeletal muscle, including myofiber nucleus-based pathobiology, which has been recently identified in the CNS of mouse models of ALS (Gertz et al., 2012), as well as faulty RNA processing in skeletal muscle.